Ag2ZnSn(S,Se)4 (AZTSSe) is an attractive photovoltaic absorber material. It is based on Cu2ZnSn(S,Se)4 (CZTSSe), but Ag is substituted for Cu to suppress bulk defects. See, for example, U.S. patent application Ser. No. 14/936,131 by Gershon et al., entitled “Photovoltaic Device Based on Ag2ZnSn(S,Se)4 Absorber,” (hereinafter “U.S. patent application Ser. No. 14/936,131”).
AZTSSe is intrinsically n-type, therefore other aspects of the device must be re-optimized. For instance, molybdenum (Mo) is typically used as the back contact material for CZTSSe photovoltaic devices. However, Mo is non-Ohmic to AZTSSe. Therefore, an Ohmic back contact is needed for the formation of AZTSSe thin film photovoltaic devices.
Further, a low-work function material is needed for making Ohmic contact to AZTSSe. This is due to well-understood physical principles involving Fermi level equilibration between the metallic contact and a semiconductor; to first order, low-work function materials tend to make Ohmic contact to n-type semiconductors whereas high work function materials tend to make Ohmic contact to p-type semiconductors. However, most low-work function metals are highly reactive with sulfur and selenium. For example, an aluminum back contact cannot be used because the annealing step may result in the complete consumption of the aluminum metal or else the formation of an unwanted aluminum selenide interlayer. Therefore, a stable low-work function contact material for AZTSSe thin film photovoltaic devices is needed.